This invention relates to a ternary metal matrix composite comprising a metal matrix reinforced with an insoluble ceramic and a semi-metal. The ternary metal matrix composite of the present invention has a lower, more consistent coefficient of thermal expansion than binary composites using similar materials, while exhibiting low density and good formability.
Composite materials are well-known and commonly used. A composite may be defined as a man-made material in which two or more constituents are combined to create a material having properties different than those of either constituents. One form of known composite is a binary metal matrix composite, which consists of a metal matrix and a reinforcement material distributed in, and bound together by, the matrix. The reinforcement usually is in the form of fibers, whiskers or particles. Typical metal matrix binding materials include aluminum, cobalt, iron, magnesium, titanium, or alloys of these materials. Typical reinforcement materials include ceramics, such as silicon carbide, boron carbide, aluminium oxide, tungsten carbide, and other borides, carbides, oxides, nitrides, silicides, aluminides, selenides, sulfides and germanides. These binary metal matrix composites are stronger and stiffer than conventional alloys and are particularly abrasion resistant.
For the past several years extensive research has been devoted to the development of binary metal matrix composites. As a result, it is well-known that ceramic reinforcement in metal matrices improves the properties of functional characteristics or various metals and alloys. Ceramic fibers, whiskers, or particulates are used to reinforce matrix metals to enhance the specific strength, specific modulus and the temperature service capabilities of the composites. Improvement in the specific strength is achieved both by reducing the density and by increasing the absolute strength and modulus through the introduction of the ceramic reinforcement. The result is typically a composite providing significant weight reduction yet meeting critical strength or stiffness requirements. These known binary metal matrix composites, however, do not have a sufficiently low coefficient of thermal expansion and/or good thermal conductivity for many applications, particularly in the electronics industry.
These is a need for a material which has a low coefficient of thermal expansion, low density and good thermal conductivity to displace heavier materials which are currently used in electronic substrates, hermetic microcircuit packages, chip submounts, heat sinks and heat pipes. Other applications for such a material include optical substrates (e.g. mirrors) where low expansion, light weight and dimensional stability are important.
In the high-reliability-electronics industry, a metal core is often employed to constrain the epoxy-glass of circuit boards which have surface mounted ceramic components. This metal core is generally composed of copper-clad Invar, molybdenum, or copper-tungsten composites. The overall weight of these materials makes them undesirable for aeronautical and aerospace applications and other uses where light weight is critically important. The metal core of the circuit boards is required to have a low coefficient of thermal expansion to reduce the stresses on the solder joints bonding the ceramic components to the printed circuit board. Without a low coefficient of thermal expansion core, the solder joints could fall due to the differences in the expansion of the ceramic component and the epoxy-glass circuit board. The metal core of the circuit board also serves to remove heat from the circuit. Thus, for example, because Invar is a very poor thermal conductor, it must be clad with copper which increases cost and limits availability. Moreover, Invar, tungsten and molybdenum are heavy materials. Molybdenum has a density of 10.2 g/cc and Invar 8.4 g/cc. The thermal conductivity is about 150 W/mK for copper-clad Invar and 160 W/mK for molybdenum.